Solution pH is widely conceptualized as a master variable in the regulation of natural aqueous systems. It is a key feature in descriptive models of carbonate system chemistry, trace metal speciation and bioavailability, oxidation-reduction equilibria and kinetics, biologically induced carbon system transformations, and the aqueous interactions and transformations of minerals. Rising levels of atmospheric CO2 are leading to ocean acidification. The response of seawater and freshwaters to acidification processes has created a need for autonomous global monitoring of ocean water and fresh water pH. The importance of pH in investigations of terrestrial and oceanic biogeochemistry has necessitated improvements in not only the quality of measurements (precision and accuracy), but also the spatial and temporal resolution of measurements in the field. Potentiometric devices are rarely used for in situ environmental pH measurements because in situ buffer calibrations are problematic. Spectrophotometric devices have been developed for in situ environmental measurements but the endurance and measurement frequency of spectrophotometric devices is relatively low due to high power requirements and the limited longevity of lamps. Achieving meaningful spatial and temporal measurements in the field mandates the introduction of robust measurement devices capable of accurate and precise measurements over extended timeframes.